Oil wells are drilled ordinarily through use of a rotary drill mechanism which advances a drill bit through the earth's formations. The cuttings are washed away by drilling mud. The drilling mud is circulated through the drill stem and flows back up through the annular bore to the surface. The drilling mud serves a multitude of purposes including the formation of a mudcake on the side wall of the open hole. The drilling mud can be readily described as an aqueous solution, typically including barites, clay and other molecules. In use, the mudcake which is formed on the side wall of the well bore is desirable for a number of practical reasons, and a fluid portion of the drilling mud will penetrate into the formation. This latter portion is normally called the mud filtrate.
Formations penetrated by a well exhibit different levels of naturally occurring gamma ray radiation. In fact, this gamma ray radiation is dependent on the nature of the formation which has been penetrated. Certain measurements are taken using gamma ray radiation measuring tools to determine the nature of the formations. For instance, one technique is to measure the total natural gamma ray radioactivity of the several formations penetrated by the well bore.
A second well logging technique presently in commercial use is the natural gamma ray spectral logging technique, often used to identify clay mineralogy, faulting and other downhole variables. This technique relies on relatively careful and delicate measurements which include ratios of natural gamma ray radioactivity from thorium, uranium and potassium. If the drilling mud contaminates the well bore with any of these elements, this logging technique may well obtain ambiguous or unreliable data. There are other logging techniques which use gamma ray, neutron, or pulsed neutron detectors. While certain of these logging techniques might be less impacted by radioactive drilling mud, it is still possible that highly radioactive mud could create a sufficiently high level of background radiation that collected data would be unreliable.
Another problem that relates to drilling mud is the formation of potentially ingestable radioactive dust or gasses evolved from the drilling mud. Precaution against such ingestion applies to the drilling mud at the drilling site, and the precautions apply to drilling mud in sacks, bags, or in bulk prior to mixing at the well site.
A separate problem arising from the use of mud additives, including barite, is that they may be mined with other compounds and the purification processing is minimal. As an example, the processing of mined barite (naturally occurring barium sulfate) at least begins with mining of the barite (in naturally occurring forms). The treatment procedure involves some degree of purification, grinding to a selected particle size, washing and other purification steps. The mineral processing steps can be relatively expensive, which expense increases depending on the extent and degree to which these steps are carried out. For instance, a common step is crushing of the barite into particles screened by a range of meshes. After crushing, washing and purification steps are ordinarily expensive so they are not undertaken to totally purify barite. One risk that accompanies the minimal cost treatment is the inevitable incorporation of impurities.
The present disclosure is directed to a method for testing raw mud additives, including barites, in the field, either when mined or after crushing, with a goal of avoiding the cost of further processing if certain impurities are detected. Particularly important impurities are those emitting gamma radiation. Because the ore is susceptible to any trace impurity, the ore potentially emits gamma radiation from actinium (Ac.sup.228), lead (Pb.sup.212), thallium (Ti.sup.208), and all daughter products of the decay of thorium (Th.sup.232). Other radioactive products include uranium-238 (which decays through radium and radon), uranium-234 and potassium-40.
Most of the gamma ray activity in barite tested preliminary to the present disclosure related to the thorium-232 decay series, however, the method discussed herein is not limited to the detection of thorium-232 daughter elements. The method detects the presence of any naturally occurring gamma ray activity. In particular, the method can be used to quantify the background produced by the radioactive isotope K.sup.40 which is found in muds containing KCl. It should be recognized that the impurities found in a particular mud additive sample may depend on vagaries such as the particular formation where it was mined, depth in the formation where mined, and a number of other variables.
Processed barite, or any other mud additive, at least after grinding, does not necessarily yield information which enables easy extrapolation of background radiation produced by mud made from the barite or by a mudcake formed in a well bore. This is highly variant dependent on many factors. For instance, it depends on the diameter of the well bore and weight of the mud. It also depends on the thickness of any mudcake which might be formed which, in turn, is a function of many other variables. The mudcake is the, more or less, solid cake formed on the side wall of the well bore; it is left there after fluid, from the drilling mud, has percolated into the formation. The mud filtrate, may very well carry with it certain radioactive elements which add to the background radiation observed in the well bore. Normally, so many factors are involved that it is not possible to routinely simply measure the radioactive levels adjacent to a bag or container of ground barite and thereby determine the background radiation level which will occur in a well.
Measurements at the pile or accumulation of ground barite are also important for environmental and local health requirements. For instance, a radioactivity level of about 2 mR/hr is the maximum level above which pile isolation is required. At an accumulation of several thousand tons of barite, radioactivity levels around the surface of the pile are not necessarily indicative of the radiation actually experienced in the well from the mud. Nevertheless, a problem is posed, namely whether or not the barite can be used if radioactivity measurements near the surface indicate some level of radioactivity. In a large mine or processing plant, the raw material is often mined and processed continuously, always altering the stored weight material and the potential of radioactive trace elements. The use of barite having trace radioactive elements adds to the background radiation observed in a well bore.
The foregoing sets forth some of the problems encountered in the use of barites in drilling mud and the impact that the drilling mud has on open hole logs which are run during well drilling with a view of proper completion. The same problems relate to other weight materials. Nothing has been said about tests run after the hole has been cased for radiation levels measured through the metal casing in the hole. The present invention is also concerned with leaving the open hole with a minimum background level so that, even after the well has been cased, the radiation levels are held to a minimum, thereby enabling well logging with radioactivity techniques years thereafter.
In addition to barite testing, other mud weight materials are also susceptible to these test procedures. A popular weight material is potassium chloride (KCl) which contains the radioactive isotope K.sup.40.